The present invention relates generally to gas engine driven compressor stations which are used to transport natural gas. More particularly, the present invention concerns a method and apparatus for increasing the operating efficiency of natural gas compressor and engine units.
Until recently, the cost of natural gas did not represent a major portion of the daily operating cost for natural gas engine driven compressors such as are used in pipe lines to transport natural gas. However, in the past few years the cost of fuel gas has increased dramatically. The increased cost of natural gas serves to emphasize the necessity to conserve the quantity of natural gas consumed by gas driven compressors which transport the gas to the market. Improving the efficiency of such gas compressor and engine units would not only avoid excessive costs but also a needless drain on the limited natural energy resources of the nation.
Generally speaking, prior art control methods and systems were not concerned with maximizing the energy efficiency of a gas compressor and engine as a unit. The control methods and systems were directed to such concerns as maintaining a desired gas flow, maintaining a certain discharge pressure from the compressor, reducing engine vibration, preventing an excessive differential pressure across the compressor, and maintaining a desired pressure to the consumer. Typically, the speed of the engine driving the gas compressor would be set at the rated speed supplied by the manufacturer. Then variations in the engine speed would be used to effect minor changes in the volume of natural gas being transported or to reduce excessive engine vibration. The torque on the engine was generally not considered as a control parameter, except for preventing an excessive brake mean effective pressure. The compressor load setting would be selected on the basis of the characteristic horsepower curves for the compressor, so that the available horsepower supplied by the engine would be utilized. Examples of prior art methods and systems are taught in U.S. Pat. Nos. 4,119,391, A. Rutshtein et al., Oct. 10, 1978; 3,753,626, R. M. Bacchi, Aug. 21, 1973; 3,716,305, G. Oberlander, Feb. 13, 1973; 3,291,378, J. P. Yarnall, Dec. 13, 1966; 3,251,534, H. E. Strecker, May 17, 1966.
In the present invention, the control parameters for the compressor and engine are selected so as to minimize the use of natural gas, i.e. the compressing energy required, to maintain the desired gas flow through the pipeline. Major losses in efficiency arise from unduly light loading of the compressor, low torque on the engine, low compression ratio, and/or high engine speed. Other factors which affect efficiency are the ambient temperature, the suction gas temperature, gas heating value, and the age or overhaul date of the engine and compressor.
The present invention further provides a control method wherein one or more gas compressor and engine units can be adapted for multiple stage compression operation. These control methods are based upon adjusting various control parameters in response to a change in one or more operating parameters in a gas compressor and engine unit. The control parameters are adjusted so that a maximum energy quotient for the unit is achieved. The use of the energy quotient value also allows for precise predictions of impending unit failures.
Additional features and advantages of the present invention will be apparent from the following disclosure taken in conjunction with the accompanying drawings and appended claims.